1. Field of the Invention
This invention relates to a shaped porous article of an aromatic polyimide having excellent heat resistance and mechanical properties and further being able to be cut with no difficulty. This invention further relates to a method for continuously preparing on an industrial scale a shaped long porous article of aromatic polyimide by ram-extrusion process using a specific aromatic polyimide powder.
2. Description of Prior Arts
It has been regarded difficult to form thick shaped articles of an aromatic polyimide because the aromatic polyimide is not thermoplastic and not melted by heating.
As a method of forming shaped articles of aromatic polyimide using an aromatic polyimide powder, a method disclosed in, for example, Japanese Patent Publication No. 49(1974)-5737 has been proposed and actually practised. The method is characterized in that an aromatic polyimide powder is compressed at a pressure in the range of 700 to 7,000 kg/cm.sup.2 and at a temperature in the range of room temperature to 400.degree. C. to form a pressed article, and the resulting article is converted into a shaped article in which particles of the powder are bonded to each other through fused phase, by heating at a temperature in the range of 300.degree. to 500.degree. C. without pressure.
In the above known method, however, a large-sized and powerful compression press is required to carry out the preparation of the pressed article under extremely high pressure. Further, it is difficult to produce a sufficiently long shaped article such as a rod having a length of more than approx. 1 meter on industrial scale. Furthermore, the above method comprises steps of filling and compressing the powder, taking out a preformed article and heating the article. These steps should be performed individually and sequentially to produce one article. Therefore, conticuous forming is substantially impossible and productivity is not satisfactory.
A method for continuous preparation of a long shaped article of a polyimide or polyamide resin is described in U.S. Pat. No. 4,238,538. This method uses an apparatus including a die with compaction, back pressure and relief zone and comprises ram-extruding the resin at a temperature of between about 20.degree. C. and 400.degree. C., but below the second order glass transition temperature of the resin to form a shaped article, followed by heating at a temperature above about 250.degree. C., for a time sufficient to coalesce the resin.
Another method for continuous preparation of a long shaped article of a polyimide is described in U.S. Ser. No. 888,929 filed by the present applicants.